Ever since the term AJAX was coined, everyone is coming out with a framework for making AJAX development easier. Some proprietary, some open source. Picking the right framework, if using one at all, depends on your personal programming style.

Whenever you use a framework, you tend to give up some control over the small details. This usually isn't an issue, but sometimes can lead to problems. When you first get started, it's probably not as important to know all of the little details going on underneath, but as you start writing more complicated solutions you will want to learn the little intricacies.

The open source community has really grabbed on to the idea of AJAX driven websites, and as a result, has produced many AJAX frameworks for open source technologies. The Microsoft folks, and ASP.NET programmer saw this happening, and recently there have been stream of AJAX frameworks coming out for ASP.NET.